1. Field of the Invention
The present invention relates to an anode active material containing tin (Sn), carbon (C) and the like as an element, and a battery using it.
2. Description of the Related Art
In recent years, many portable electronic devices such as a combination camera (Videotape Recorder), a mobile phone, and a notebook personal computer have been introduced. Downsizing and weight saving of such devices have been made. Regarding a battery used as a portable power source for such electronic devices, particularly a secondary battery, active researches and developments for improving an energy density have been actively promoted as a key device. Specially, a nonaqueous electrolyte secondary battery (for example, lithium ion secondary battery) can provide a higher energy density compared to a lead battery or a nickel cadmium battery, the traditional aqueous electrolytic solution secondary battery. Therefore, improving the nonaqueous electrolyte secondary battery has been considered in various circles.
As an anode active material used for the lithium ion secondary battery, a carbon material such as non-graphitizable carbon and graphite, which shows a relatively high capacity and has good cycle characteristics is widely used. However, considering that a higher capacity has been requested in these years, there is a task to obtain a higher capacity of the carbon material.
Based on such a background, a technique to attain a high capacity by a carbon material by selecting a raw material for carbonization and preparation conditions has been developed (for example, refer to Japanese Unexamined Patent Application Publication No. H08-315825). However, when such a carbon material is used, the anode discharge potential to lithium is from 0.8 V to 1.0 V, and the battery discharge voltage when the battery is made becomes low. Therefore, in view of the battery energy density, significant improvement is not prospective. Further, there is a disadvantage that large hysteresis is shown in the charge and discharge curve, and the energy efficiency in each charge and discharge cycle is low.
Meanwhile, as an anode with a higher capacity than using the carbon material, researches on an alloy material obtained by applying the fact that a certain metal is electrochemically alloyed with lithium (Li), and the alloy is generated and decomposed reversibly have been promoted. For example, a high capacity anode using Li—Al alloy has been developed, and further, a high capacity anode made of Si alloy has been developed (for example, refer to U.S. Pat. No. 4,950,566).
However, the Li—Al alloy or the Si alloy is swollen and shrinks according to charge and discharge, and the anode is pulverized as every charge and discharge is repeated. Therefore, there is a large disadvantage that the cycle characteristics are significantly poor.
Therefore, as a technique to improve cycle characteristics, coating the surface of the alloy material with a material having high conductivity has been considered (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2000-173669, 2000-173670, and 2001-68096). In the techniques described in these patent documents, the alloy surface is coated with a conductive material by soaking the alloy material in the organic solvent in which a conductive material is dissolved or by a technique using mechanochemical reaction such as hybridization to improve cycle characteristics.